1. Field of the Invention
The invention relates to a method for the secure storage of information worth protecting, which can contain a coding method and a decoding method. Furthermore, the invention relates to a coder/decoder (“codec”) for implementing the method and to a data storage medium for the holographic storage of encrypted data, which can be produced by the coding method.
2. Description of Related Art
In order to protect information, it is generally converted into a digital format and then encrypted by means of a cryptographic encryption method in such a way that it can be decrypted substantially only with knowledge of the key used. In the case of good encryption, the time expenditure for finding the key to decrypt the encrypted information with the aid of a computer is so high that an attempt to carry out decryption becomes unattractive to unauthorized persons. The reason for this is that trying out various keys (“brute force attack”), for example by different passwords being tried out repeatedly, lasts a very long time in the case of a password having many digits and a correspondingly high entropy. The disadvantage in this case, however, is that the encrypted information present in digital form can be copied many times, so that, for example, it can be stored on many different computers via the Internet, it being possible for another range of possible keys to be tried out on each computer (parallelized attack).
In order to prevent copying of the encrypted information, it is known to provide copy protection. For example, EP 1 355 217 discloses providing hidden features in the storage medium on which the encrypted information is stored, which features have to be checked before the actual data are read out. During the copying of the data storage medium, the hidden feature is overlooked as not belonging to the file, so that the feature is not copied as well and, as a result, the copy is not complete. However, this copy protection can be circumvented by a copying program not only copying regions of a storage medium identified as data but all the regions of the storage medium, irrespective of whether they are identified as used or unused regions. Despite copy protection, it is therefore possible to prepare as many copies of the encrypted information as desired, in order to have them decrypted by a plurality of different computers.
U.S. Pat. No. 3,894,756 discloses the provision of holographic data storage in which, during the production of the hologram, an object beam has a randomly modulated reference beam superimposed on it. This leads to optical encryption of the information to be stored. The same modulation must be used when reading. The disadvantage with this coding method is that, in order to provide the modulated reference beam, a mechanism having movable components is required which is accordingly susceptible to faults, not least on account of wear phenomena and the requirement for particularly exactly controlled positions. Even a single slight error during the creation of the hologram, which, for example, can arise as a result of a slight knock against the apparatus used, leads to damaged information, which can no longer be reconstructed even by an authorized user.
The object of the invention is to provide a coding method, a decoding method, a codec and a data storage medium for holographic storage which presents greater resistance to a parallelized brute force attack and is simple to handle with a reduced susceptibility to faults.